LEDs are generally manufactured from elemental structures corresponding to a stack of layers comprising at least one n-type layer or region, one p-type layer or region and an active layer disposed between the n-type and p-type layers.
As for solar cells, they are manufactured from elemental structures comprising at least one p-n junction (junction of a p-type layer and an n-type layer). These elemental structures can include a plurality of p-n junctions. As is well-known to those skilled in the art, a p-n junction contains an active zone corresponding to the space charge region (SCR) located around the junction.
The elemental structures mentioned above can be formed from the same growth substrate on which a stack of the necessary layers is formed by epitaxial growth, portions of this stack then being cut out of the substrate to insulate the elemental LED or photovoltaic structure.
However, other LED or solar cell manufacturing operations, such as wiring by the formation of n and p contact pads, or disassembly/removal of the growth support required to carry out subsequent treatments, are carried out all or in part on the level of each elemental structure individually, meaning that the elemental structures are separate from each other and that one structure is treated at a time.
The same is true for operations involved in the assembly of LEDs or solar cells on a mechanical support or for operations of depositing a light-converting material (“phosphorus”) carried out individually for each device.
FIG. 1A schematically represents an elemental LED structure 3 obtained after cutting out a growth substrate (sapphire, for example) comprising a plurality of identical LED structures. The elemental LED structure 3 is composed of a stack of an n-type layer 4, an active layer 5 and a p-type layer 6. This elemental LED structure 3 is formed on a growth substrate 2 and further includes, on the upper surface of the p-type layer 6, a reflective layer (mirror) 7, the whole of which forms a multi-layer structure 1.
As is known, the multi-layer structure 1 is then assembled on the exposed surface of the mirror layer 7 with a wafer bonding substrate 8 (FIG. 1B). Traditionally, it is of use to prepare this assembly by thermocompression bonding, this bonding requiring the application of a certain pressure and a particularly high temperature (above 300° C.) in order to guarantee the robustness of the assembly. For example, this bonding can be carried out using a gold-tin alloy enabling soldering between the two surfaces to be bonded.
Once the assembly is complete, the growth substrate 2 (acting as a temporary substrate) is removed from the rest of the multi-layer structure 1, the procedure for such a removal being well-known to the person skilled in the art (FIG. 1C).
The Applicant has, however, observed several major disadvantages related to the thermocompression bonding technique.
The increase in temperature during thermocompression bonding leads to significant thermal expansion of the growth substrate 2, as well as the final substrate 8, this dilation being a function of the respective coefficient of thermal expansion (CTE) of the substrates 2 and 8. To obtain satisfactory bonding results, it is necessary to choose the type of substrates 2 and 8 so that they are compatible in terms of CTE with the LED structure 3. A CTE mismatch that is too great is likely to lead to fractures and, consequently, to reduce the manufacturing yields of the structures in question.
The high temperature during bonding further generates deformations of the growth substrate (bowing, warping). This deformation phenomenon is particularly amplified when the growth substrate of the structure to be bonded is large (150 or 200 mm, for example). It is then necessary to apply a greater pressure during assembly in order to limit these deformations. Consequently, the current practice tends to bond each LED structure individually on the final substrate in order to minimize mechanical stress during thermocompression bonding.
These CTE compatibility constraints considerably limit the choice of materials that can form substrates 2 and 8. The choice can, for example, be concerned with germanium, which has the disadvantage, however, of being expensive and relatively unavailable on the materials market.
There is a need for a technique for manufacturing structures of LEDs or of solar cells that is effective and makes it possible to be freed from the constraints and disadvantages mentioned above.